The present invention relates to a magnetic resonance imaging (MRI) apparatus, in particular, an RF (radio frequency) coil for obtaining magnetic resonance signals.
MRI apparatus is medical diagnostic imaging apparatus which induce magnetic resonance in nuclei in an arbitrary cross-section passing horizontally through a test subject, and obtaining a tomogram in the section from generated magnetic resonance signals. In general, an RF magnetic field is irradiated by an RF coil to excite hydrogen nuclei (1H), and generated magnetic resonance (RF magnetic field) signals are detected.
An RF coil irradiates an RF magnetic field, and receive magnetic resonance signals. This coil was constructed with parallel resonance circuit or series resonance circuit each having capacitors and inductors. Each resonance circuit is tuned the same frequency as the nuclear magnetic resonance frequency f0 of the nuclei. (refer to, for example, MURPHY-BOESCH, et al., “An In Vivo NMR Probe Circuit for Improved Sensitivity”, Journal of Magnetic Resonance, USA, 1983, 54, p.526-532). Since the nuclear magnetic resonance frequency f0 becomes higher in proportion to the magnetic field intensity, the RF coil used for an MRI apparatus using a high magnetic field must be designed so that it should resonate at a high frequency. At present, as shown in FIG. 20A, capacitors are inserted into the loop of the RF coil to reduce the combined capacity of the capacitors, and thereby increase the resonance frequency.
In general, in order to efficiently convey power to the RF coil from a cable, it is necessary to attain impedance matching between the cable and the RF coil. Moreover, in order to efficiently generate an RF magnetic field with the RF coil, it is necessary to make phase of 0 degree. Therefore, the RF coil is designed so that impedance matching should be attained, and phase should become 0 degree at a magnetic resonance frequency f0 which is a resonance frequency fR of the RF coil. The impedance is adjusted to, for example, 50Ω. As shown in FIG. 20B, frequency characteristics of phase and impedance when the RF coil of FIG. 20A is adjusted as described above. Where fR is resonance frequency.
However, if a test subject is placed in an RF coil, loss is caused in the RF coil by the coupling of the test subject and the RF coil. And the frequency characteristic of the coil changes. The frequency characteristic in such a case is shown in FIG. 20C. When loss is caused in the RF coil, the impedance falls at the resonance frequency fR, and change of the phase around the resonance frequency fR becomes mild as shown in FIG. 20C. As a result, the impedance shifts from 50Ω at the magnetic resonance frequency f0, which is the resonance frequency fR, and the phase is also no longer 0 degree. Therefore, the RF coil cannot generate and detect signals at the magnetic resonance frequency, and sufficient sensitivity cannot be obtained.
To solve this problem, when a test subject is inserted in an RF coil, value of the capacitor is conventionally readjusted to make values of the impedance and phase 50Ω and 0 degree, respectively, at the nuclear magnetic resonance frequency f0 to obtain magnetic resonance signals.